1. Field of the Invention
The present invention is broadly concerned with a method of forming solid, non-edible biodegradable articles such as eating utensils, cups, plates, sheet items, packaging, and other convenience products. More particularly, the invention pertains to such methods and the resultant formed articles wherein a formulation including naturally occurring grain protein, starch, plasticizer and a reducing agent operable for cleaving disulfide bonds present in the grain protein as well as optional ingredients, lubricants, fillers, and mold releasing agents is prepared and heated under relatively low temperature conditions in order to render the formulation substantially homogeneous and flowable; the formulation can then be injection molded, extruded or otherwise formed to give complete biodegradable articles.
2. Description of the Prior Art
Petroleum-based synthetic resins have achieved widespread use in the fabrication of a multitude of products. To give but one example, single or limited use items such as eating utensils and cups are commonly produced using synthetic resins, e.g., polyethylene, polystyrene or polypropylene. While such items are nominally "disposable", in reality they are largely indestructible owing to the long-lived properties of the synthetic resin. As a consequence, there is a growing concern about the indiscriminate use of petroleum-based synthetic resins, and their accumulation in the environment.
One response to this problem has been the development of biodegradable plastic materials which degrade in a relatively short period of time under normal environmental or composting conditions of temperature, humidity and the action of microorganisms. Research in this area was quite active in the 1930s and 40s, particularly in the context of soybean-derived plastics. At that time, soybean products were incorporated into phenolic resins as a filler or extender, and to enhance biodegradability. Subsequent research has led to products having improved biodegradation rates. For example, products have been developed which employ starch as fillers in petroleum-based plastics. In addition, inherently biodegradable polymers have also been proposed using starch and plant proteins as primary ingredients while essentially avoiding synthetic resins. The latter class of biodegradable products are particularly attractive, inasmuch as plant protein sources are relatively low in cost, renewable, and are readily available. Many of the processes involving wholly biodegradable plastics have involved compression molding.
Thus, extensive work has been done on the processing of gelatinized starch into molded articles. A major problem with starch-based products is water sensitivity. In an attempt to overcome this obstacle, it is known to blend starches with other synthetic resin polymers, to modify the starch to make it more hydrophobic, or to convert starch into glucose and fermenting the glucose into monomers suitable for use in preparing biodegradable polymers.
Grain proteins are one family of biomaterials which have received relatively scant attention as raw materials for wholly biodegradable products. Grain proteins normally exhibit better moisture resistance than starch, but exhibit significant processing problems such as altered rheology and flow characteristics, especially under conditions of heat denaturation. As a consequence, it can be very difficult to economically process grain protein-based plastics using conventional extrusion and injection molding equipment. These problems are believed to stem in part from the highly branched and networked structure of grain proteins and their ease of denaturation under normal processing temperatures.
U.S. Pat. No. 3,615,715 describes the production of non-edible sausage casing films from nonheat coagulable proteins using an extrusion device. An essential feature of the production process disclosed in this patent is time-temperature and moisture conditions which yield denatured film products.
There is accordingly a need in the art for improved techniques for forming biodegradable articles which essentially avoid environmentally deleterious synthetic resins but permits economical fabrication of finished articles using high speed extrusion and injection molding equipment.